Field of the Invention
The present disclosure relates to wireless power transfer by electromagnetic coupling. It more specifically relates to a transmission device capable of simultaneously electrically powering a plurality of electronic devices.
Description of the Related Art
Systems where an electromagnetic field provided by a transmission device (transmitter) is used to electrically power an electronic device (receiver) are known. The receiver for example is a lamp, a wireless phone, an electric toothbrush, an electronic tag, a medical implant, or any other device capable of being remotely supplied. The transmitter comprises an A.C. signal generator connected to a conductive winding forming an antenna, and electronic components such as capacitors or resistors performing frequency tuning and impedance matching functions. The receiver comprises a conductive winding forming an antenna and components performing frequency tuning, impedance matching, and rectification functions. To optimize the power transfer, the antenna circuits of the transmitter and of the receiver should be sized according to the receiver. The main parameters are the number of turns of the conductive windings forming the transmit and receive antennas, the dimensions of the antennas, and the distance between the transmit antenna and the receive antenna.
As an example, in a conventional representation, the transmitter is modeled as a series resonant circuit RELECE connected to an A.C. signal generator, and the receiver is modeled as a parallel resonant circuit RRLRCR. Resistance RE corresponds to the resistance of the antenna circuit of the transmitter, that is, to the sum of the specific resistance of the antenna and of a possible added impedance matching resistance, inductance LE corresponds to the inductance of the transmitter antenna, and capacitor CE corresponds to a transmitter resonance frequency tuning capacitance. Resistance RR corresponds to the load formed by the receiver device on the antenna circuit of the receiver, inductance LR corresponds to the inductance of the receiver antenna, and capacitor CR corresponds to a capacitance for tuning the receiver resonance frequency.
In practice, the voltage level obtained, on the receiver side, across capacitor CR, depends (among other parameters, such as the size of the antennas, their characteristics, and the transmit power) on the distance between the transmit antenna and the receive antenna. The relation between the distance between antennas and the voltage level on the receiver side is generally not monotonous. The transmit and receive circuits are sized to allow an optimal power transfer at a nominal distance between the receiver and the transmitter.
Systems where a magnetic field provided by a transmitter is used to simultaneously electrically power a plurality of receiver devices are more specifically considered herein.
A system where the transmitter comprises a single antenna powering a plurality of receivers has already been provided. Such a system has the advantage of having a simple design, but does not allow an optimized power transfer between the transmitter and the receivers.
A system where the transmit device comprises a plurality of transmit antennas, each antenna being specifically tuned with a receiver, has also been provided in patent application US20090140691. The transmit device comprises control and switching circuits enabling to active/deactivate certain antennas. Further, wireless communication means are provided between the transmit device and the receivers, enabling receivers to notify their presence to the transmit device. When the transmit device detects the presence of a receiver, the corresponding antenna circuit of the transmit device is activated. When the receiver is drawn away from the transmit device, the corresponding antenna circuit is deactivated. Such a system allows an optimized power transfer between the transmit device and each of the receivers, but has the disadvantage that the transmit device is relatively complex.